Compounds reactive with biomolecules such as antigens, antibodies, and/or DNA-segments with the corresponding complimentary species for measurements of enzyme kinetics, receptor-ligand interactions, nucleic acid hybridization kinetics in vitro as well as in vivo, etc. are useful. Such compounds are of interest for the pharmacological characterization of receptors and drugs, as well as other uses. For such applications, xanthylium salts (e.g., as disclosed in U.S. Pat. No. 5,846,737) or cyanines (e.g., as claimed in U.S. Pat. No. 5,627,027) had been used, but had the disadvantage of tending to aggregate and to form dimers, especially in aqueous solution, due to planarity of their π-system. Further, labels with insufficient hydrophilicity undergo non-specific interactions with various surfaces, which cause problems in purifying the corresponding conjugate and lead to an unsatisfactory signal to noise ratio.
Recent efforts have been directed to reduce the known disadvantages of cyanines by introducing additional substituents that increase the hydrophilicity of the compounds. Additional sulfonic acid functions have been introduced into the cyanine chromophore. For example, U.S. Pat. No. 6,083,485 (Licha) and U.S. patent application Ser. No. 09/968,401 and Ser. No. 09/989,853 (Molecular Probes) disclose cyanines in which one of the common methyl groups in the 3-position of the terminal indole heterocycle is substituted by an ω-carboxyalkyl function and in which the previously present (e.g. in Cy3 or Cy5) N-alkyl or N-ω-carboxyalkyl functions are replaced by N-ω-alkyl sulfonic acid functions. Published PCT Application No. WO 05/044923 discloses cyanines in which the common methyl substituent in the 3-position of the terminal indole heterocycle is substituted by a N-ω-alkyl sulfonic acid function. Cyanines with more than two sulfonic acid functions disclosed in these documents exhibited higher solubility and, in connection with this, lower tendency to dimer formation, in comparison to cyanines (Cy3, Cy5) described in U.S. Pat. No. 5,627,027.
Further improvements are desirable.